1. Field of the Invention
The invention relates generally to methods for producing doped glass. More specifically, the invention relates to a method for producing a titania-doped fused silica glass and a titania-doped fused silica glass having low water content.
2. Background Art
Extreme ultraviolet (EUV) lithography is emerging as one of the next-generation lithography techniques that will allow high-volume production of integrated circuits with sub-100-nm features. EUV lithography as currently contemplated involves producing electromagnetic radiation at around 13 nm. The EUV radiation may be produced, for example, using a 1064-nm neodymium-YAG laser which produces a xenon gas plasma or from a synchrotron source. A condenser collects the EUV radiation and projects it onto a mask containing a pattern to be replicated on a silicon wafer. The mask reflects the EUV radiation into an imaging system, which then projects an image onto a resist-coated silicon wafer. The pattern is later transferred to the silicon wafer by etching.
The mask structure consists of a substrate (“mask blank”), a reflective multilayer stack formed on the mask blank, and an absorber formed on the multilayer stack. Typically, the multilayer stack includes alternating layers of Mo and Si or Mo and Be. The absorber defines the pattern to be replicated on the silicon wafer. The mask blank may be made of silicon or glass or other suitable material. It is important that the mask blank has a low thermal expansion so that it does not distort under exposure to the EUV radiation. Titania-doped fused silica (SiO2—TiO2) is one example of a glass that can be made to have a very low thermal expansion, i.e., lower than pure fused silica with the potential for a coefficient of thermal expansion that approximates zero. The coefficient of thermal expansion of the SiO2—TiO2 glass can be controlled by adjusting the percent weight content of TiO2 in the glass.
Commercial processes for producing SiO2—TiO2 glass involve transporting a mixture of a silica precursor and a titania precursor to a reaction site, thermally decomposing the mixture of precursors (usually via flame hydrolysis) into SiO2—TiO2 particles (“soot”), and depositing the soot on a support. In the conventional boule process, the soot is captured in a cup of a refractory furnace at consolidation temperatures (typically 1200 to 1900° C.) so as to allow the soot to immediately consolidate into a solid body (“boule”). These high consolidation temperatures may result in compositional variations within the glass, which would result in the glass having non-uniform thermal expansion properties. Applications such as EUV lithography require very low variations in coefficient of thermal expansion (CTE) within the substrate (e.g., 0±5 ppb/° C.). Therefore, a production method which favors homogeneity in the SiO2—TiO2 glass is desirable.
For environmental reasons, commercial processes for producing SiO2—TiO2 glass use a chloride-free material such as octamethylcyclotetrasiloxane (OMCTS), a siloxane, as a silica precursor. Usage of organic precursors and a hydrogen-containing fuel for thermal decomposition of the organic precursors inherently results in the SiO2—TiO2 glass containing more OH (often referred to as water) than can be tolerated by infrared transmission applications or deep-UV applications such as at 157 nm. In particular, OH has some absorption at these wavelengths. Therefore, a production method which favors dehydration of the SiO2—TiO2 glass is also desirable.